Claims such as 'a standard box keeps the cold 18-24 hours' end with 'under the right conditions' but do not give those conditions as numbers. The heat balance below separates the duration from the conditions.
What a box does and does not do
| It does | It does not |
|---|---|
| Slows heat transfer | Produce cold |
| Reduces the effect of ambient conditions | Guarantee a duration |
| Protects the product physically | Prevent freezing |
Calculation: 30 mm EPS, 30 °C outside
The inputs are examples: outer size 420 × 320 × 320 mm, 30 mm wall, surface about 0.74 m², inside temperature 5 °C. For EPS the conductivity used is λ = 0.035 W/(m·K). The BEWI EPS data sheet gives 0.030-0.038 W/(m·K), and this value sits in the middle of that range. Heat entering the box is Q = λ × A × ΔT / d.
Q = 0.035 × 0.74 × 25 / 0.030 ≈ 21.6 W. Over 24 hours that is 21.6 W × 86,400 s ≈ 1.86 MJ. Ice melting at 0 °C absorbs 334 kJ/kg (the heat of fusion of water, 334 J/g), so 1.86 MJ / 334 kJ/kg ≈ 5.6 kg of ice.
| Outside temperature | ΔT (K) | Heat entering the box (W) | Over 24 h (MJ) | Ice at 0 °C needed (kg) |
|---|---|---|---|---|
| 20 °C | 15 | 13.0 | 1.12 | 3.4 |
| 25 °C | 20 | 17.3 | 1.49 | 4.5 |
| 30 °C | 25 | 21.6 | 1.86 | 5.6 |
| 35 °C | 30 | 25.9 | 2.24 | 6.7 |
The table counts only the heat of fusion. It does not include the warming of frozen packs, the heat capacity of the box and its contents, or the corners and lid seams. The result is therefore a first estimate; the exact value comes from a thermal test on your own route.
A statement that the box holds 24 hours can only be made once the ice mass, the outside temperature and the wall thickness are written with it. On a 35 °C summer route the same 24 hours needs 6.7 kg of ice.
Box life and records
WHO requires reusable shipping container systems to be re-qualified periodically, because age, damage, evaporation of temperature stabilizers and similar factors can reduce performance. The usual practice is once a year or after a significant change, based on risk assessment. A change of components, route or duration calls for re-evaluation (WHO TRS 961 Annex 9, Supplement 13, section 2.1.4 and the re-qualification section).
Minimum to do
- a thermal test of the box on your own route
- a written packing instruction: how many packs, where, with which barrier
- a logger beside the product on every shipment
- a record whenever the instruction is departed from